Localization of radio-frequency transceivers

ABSTRACT

The invention concerns a localization and communication methods and system between mobile stations and a central server through a wireless network comprising a plurality of wireless radio-frequency transmitting access points (AP 1 -AP 4 ), among which a first access point is chosen to perform the communication, comprising the steps of measuring the signal strengths received by said station from the plurality of access points; storing each measured strength with an address identifying the corresponding connected access point; comparing said stored strengths to values of a predetermined table of signal strength thresholds affected to access points, defining one or more event zones (EVA, EVB) each comprising one or more attenuation ranges of one or more access points; and considering the station as located in a given event zone if the measured strength corresponding to an access point defining that event zone is comprised in the attenuation range of that access point.

FIELD OF THE INVENTION

The present invention generally relates to a communication method and asystem between a central server and mobile transceivers, calledhereafter “stations”, through a radio-frequency (RF) wireless network.More particularly, the invention relates to such a system able to workindoors or in public places.

BACKGROUND

In an indoor or public space environment, there is a need for acommunication system able to dedicate applications (for example, accessto specific services) on the basis of the location of a user (moreprecisely, a user station). A problem is to localize the stations in theenvironments with respect to physical areas to which dedicated proximitybased application(s) are related.

Many outdoor localization techniques are known. For example, the GPS(Global Positioning System), the triangulation from earth fixedtransceivers, the radio angle measurements, are efficient outdoorlocalization systems. However, such systems are unable to perform anindoor (in-building) localization due to the shielding resulting fromwalls and radioelements. Such perturbing elements that can also bepresent outside render the above systems not very precise even outdoors.

A first known indoor localization system uses fixed infraredtransceivers located in a building. The system determines in the fieldof which transceiver is the object and considers this field area as thelocation. A drawback of such a system is that many infrared fixedtransceivers have to be installed in the building due to the poor rangeof IR access points, and their limitations to being line-of-site.Further, such a system has to be dedicated to the localization.Additionally, the data carrier bandwidth of IR-systems is not sufficientfor most of the applications of wireless system for which the requiredtransmission rate becomes higher and higher.

Recent indoor communication systems use radiofrequency transmission.Such systems provide fixed access points to the wireless communicationnetwork, which are distributed inside the building or public space to becovered. A station is connected to the network through the access pointfrom which it receives the higher signal strength. This determination ismade by the station itself which examines the signal levels receivedfrom the different access points, and then chooses one of these tocommunicate with to access network resources. The network routingprotocol takes into account the access point choice of the station inorder to switch the communication to the right access point. Of course,such systems also work outdoors, and at least a part of the accesspoints can be located outdoors. For example, networks in accordance withthe standard 802.11b to which the present invention applies moreparticularly belong to this type of systems.

Recently, some of these systems have been provided with a localizationfunction of the stations, that is a determination of the physicallocation (and not only the determination of the access point to beconnected to).

An example of such in-building radio-frequency wireless network systemis described in the article “A Software System for Locating MobileUsers: Design, Evaluation, and Lessons” by Paramvir Bahl, Venkata N.Padmanabhan and Anand Balachandran, published in December 2000,published athttp://www.cs.ucsd.edu/users/abalacha/research/papers/msr-tr-2000-12.pdfas of November 2001. In that system, a history-based station-trackingalgorithm takes into account the motion of the user station in order tofollow this motion from an access point to another using apre-determined and configured database representing a static signalstrength map in principle.

Another example is described in the article “Determining User Locationfor Context Aware Computing Through the Use of a Wireless LANInfrastructure” by Jason Small, Asim Smailagic and Daniel P. Siewiorek,published in December 2000, published onhttp://www-2.cs.cmu.edu/^(˜)aura/publications.html. Such a system uses atriangulation technique based on signal intensity from wireless accesspoints.

U.S. 2001/036833 discloses a localization system in which the mobilestations monitor the electric field intensity received from a pluralityof repeaters. The mobile stations transmit to a central server themeasured intensities. The central server localizes the mobile stationsaccording to wave propagation pattern information contained in thecentral server.

WO 00/38460 discloses a localization system in which mobile stationstransmit identity information at a predetermined power level.Localization beacons receiving this information respond to the mobilestations only if the received signal level is higher than apredetermined threshold.

A purpose of the present invention is to provide another localizationmethod and system.

Another purpose of the present invention is to provide such alocalization method and system particularly adapted to dedicateapplication(s) to the relative location of the stations.

Another purpose of the present invention is to provide such a method andsystem which do not need structural hardware modification of theexisting mobile stations, wireless access points and central server.

Another purpose of the present invention is to provide a communicationmethod and system between mobile stations and at least one centralserver through a radio-frequency wireless network, using proximity basedapplications.

Another purpose of the present invention is to provide a method andsystem adapted to omnidirectional RF system and, more particularly, toone unregulated spectrum standard suites.

SUMMARY OF THE INVENTION

To attain these purposes and others, the present invention providesmeasuring the strengths or levels of the signal received from aplurality of access points, by a mobile station to be located, storingthe measured values with identifying physical addresses of thecorresponding access points, and matching the stored values with signalstrength thresholds of a table of event zones delimited by at least asignal strength boundary around an access point.

According to the present invention, an event zone corresponds to one ormore attenuation ranges delimited by signal strength boundary(ies)around at least one access point. One or more event zones are defined inthe wireless network environment depending on the physical location ofthe access points.

A feature of the present invention is to use, for the access points of alocal area network, preferably of the 802.11b standard, signal strengthattenuation thresholds to define, around the access points, attenuationranges which are parts of event zones, and to use these virtual rangesto determine in which event zone(s) a mobile station communicating withone of the access points is located.

Applied to an indoor environment, the invention takes benefit of thefact that, even if a wall or similar RF affector constitutes a partialscreen for the RF-field of an access point, it is not a drawback for theinvention. Indeed, in an indoor environment, the event zone boundarieswill often correspond to the wall of the building or of therooms—depending on the variable adjustable attenuation rangethreshold(s) that are included in the definition of the event zone(s).Therefore, the attenuation of the wall contributes to the definition ofthe event zones.

According to the present invention, the communication link itself is notmodified. For example, applied to a system selecting the access pointfrom which the mobile station receives the highest signal strength, thecommunication is according to the invention still made through that way,independently from the determination of the event zone(s) to which thestation belongs.

Another feature of the present invention is to launch or triggerapplications or services based on the event zone(s) in which a givenmobile station is located. In other words, having determined one or moreevent zones in which is located a station, a particular communication,which depends on the event zone(s) is established between that stationand a central server.

As for the communication link itself, the location of a given mobilestation with respect to the event zone(s) is periodically checked inorder to update the proximity based applications if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and others purposes, features, aspects and advantages ofthe invention will become apparent from the following detaileddescription of embodiments, given by way of illustration and notlimitation with reference to the accompanying drawings.

FIG. 1 is a schematic representation of the main elements of acommunication system to which the present invention applies;

FIG. 2 illustrates coverage areas and attenuation ranges of definedevent zones according to an embodiment of the present invention;

FIG. 3 is a schematic representation of an indoor environment in whichis implemented an embodiment of the present invention; and

FIG. 4 is a flow chart of an implementation of the localization methodaccording to the present invention.

For clarity, only the basic steps and elements that are necessary to theunderstanding of the present invention have been shown in the drawingsand details will be described hereafter. In particular, theimplementation of the invention by means of computerized systems willnot be detailed as being in the ability of one skilled in the art.Further, the features of a local area network corresponding to the802.11b standard will not be detailed as being well known by thoseskilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically represents the main components of a communicationsystem to which the present invention applies.

A central server (CS) 1 is made, for example, of a network computer orany conventional computerized system, able to organize and controlnetwork communications, preferably according to one unregulated spectrumstandard suites. The invention applies to network communications,preferably according to one unregulated spectrum standard, such as ISMbands. Today, ISM (Industrial, Scientific, Medical) bands are proposedat a suitable frequency allocation given that it is available globally(though the amount of spectrum in a band is not always the same in eachcountry). The ISM bands are unregulated bands—anyone can use a bandprovided the maximum ERP of any device used does not exceed 100 mW (+20dBm), with most devices operating at 1 mW (0 dBm). For example, theinvention applies to the following Wireless Local Area Networks (WLAN):802.11, 802.11a, 802.11b, 802.11e, 802.11f, 802.11g, 802.11h, 802.15.1,802.15TG2, 802.15TG3, 802.15TG4, and to the following other unregulatedspectrum standards: Bluetooth, Wi-Fi, HiperLAN1, HiperLAN2 and otheremerging or relevant WLAN, PAN, and WMAN/WANs standards.

The central server 1 is generally connected to a wired network (notshown). Mobile devices (MS) 2, called stations (for example PersonalComputers or Personal Digital Assistants equipped with wireless networkinterface cards, or phones with 802.11b dual chipsets), provided with RFtransceivers (symbolized by antennas 21), can communicate with thecentral server 1 through access points (AP) 3. The access points 3 arewired to the central server (wires 4) and provided with RF transmittingmeans (symbolized by antennas 31). Access points act as interfacesbetween the server and the stations, i.e. the wireless and wirednetworks. Other stations (not shown) are usually wire connected to thecentral server. However, the invention more particularly applies towireless mobile stations. In FIG. 1, a database (DB) 5 is connected tothe central server. Database 5 aims at containing conventional data andprograms, or retrieves data and programs or services via a network, andcorrespondence tables between the signal level thresholds associated toeach access point and the corresponding event zones of the environmentto be covered by the system according to the invention.

The access points are distributed according to the environment to becovered. In the preferred application of the invention, the environmentis, at least partially, indoor.

As in a conventional wireless network according to the 802.11b standard,a mobile station wanting to establish a communication with the centralserver selects, among the access points, the one from which it receivesthe highest signal strength or level. Each access point has an accesspoint address, which is unique in the environment. A station wishing toestablish a communication compares the received signal strengths of themultiple access points it can see. The station then registers the accesspoint that has the strongest received signal strength and uses thataccess point as its connection to the central server. The central serverthen uses the corresponding access point to exchange data with thestation. Each access point has a unique identifying address (called itsMAC address which is defined by the IEEE and controlled to be uniqueglobally). The station communicates the MAC address of its currentconnected access point to the central server to be used for thelocalization, event zone identification, and application association forthat station in that attenuation area in that event zone.

According to the invention, the signal strength data concerning a givenstation are used, on the central server side, to determine the locationof the station with respect to event zones. Event zones are, accordingto the invention, fuzzy physical areas in each of which one or moreaccess points of the networks can transmit and to which are associatedone or more applications. An application is, according to the invention,service, software or data, which is available to a station located inthe corresponding event zone.

According to a preferred embodiment of the present invention, at leastone specific application is dedicated to each event zone defined in thecorresponding environment. The specific application can be datacommunicated to the mobile station(s) in the zones, programs stored inthe central server or network which are executed for the mobile stationslocated in a given event zone, or services rendered available for themobile stations located in the corresponding event zone(s).

For example, in a bank building in which event zones corresponding tobank services are delimited, an account consulting event zone can bedefined in a physical area of the bank. When a user and its mobilestation (for example, his personal digital assistant) comes into thatarea, the central server communicates to the station the balance of thebank account of the user. Another event zone can be a stock exchangeordering zone in which the user can be connected to the order office ofthe bank. Out of the corresponding zone, the functionality is notavailable to the station.

FIG. 2 schematically illustrates an embodiment of the localization andcommunication methods according to the present invention. These methodsare, for example, implemented in a communication system as illustratedin FIG. 1.

According to the invention, the event zones are based on the signalattenuation around several access points. In other words, an event zoneis limited by a virtual boundary of signal strength around one orseveral access points. For example, a first event zone EVA comprises thephysical area corresponding to field areas of three access points AP1,AP2, AP3 for which the attenuation of the signal is, respectively, lowerthan −32 dB, −40 dB and −48 dB. In FIG. 2, the attenuation ranges havebeen shown in full lines, and the event zone EVA has been delimited withlong portions dotted line. A second event zone EVB is shown (in shortportions dotted line) and comprises the attenuation ranges lower than−48 dB of the access point AP3 and of a fourth access point AP4. Theattenuation ranges used by the invention to define an event zone can ornot be the same in terms of attenuation, for each access point of thatzone.

For clarity, the areas have been shown approximately circular in FIG. 2.However, and as it will be better understood in connection with FIG. 3,these areas may have other shapes.

FIG. 3 schematically represents an indoor environment to which thepresent invention applies.

In FIG. 3, four rooms R1, R2, R3, R4 and one corridor C of a buildinghave been shown. Access points AP10-AP19 are distributed in the indoorenvironment. The access points are, for example, distributed into theindoor environment so as to give the ability to the central server to beconnected to any mobile station coming into the environment.

According to the present invention, the signal level thresholds used asboundaries of the attenuation ranges of the access points to define theevent zones are fixed on the basis of the environment. In other words,knowing the physical map of the environment and the event zones to becreated, the system is configured to define the level thresholds of thevarious access points.

For example, four event zones EV1-EV4 are defined in the environment ofFIG. 3. These event zones approximately correspond to the differentrooms as follows:

-   -   Event zone EV1: attenuation ranges around access points AP1,        AP15 and AP16 with level thresholds chosen to correspond to        rooms R1 and R4;    -   Event zone EV2: attenuation range around access point AP12 with        level threshold chosen to correspond to rooms R2;    -   Event zone EV3: attenuation ranges around access points AP13 and        AP14 with level thresholds chosen to correspond to rooms R3; and    -   Event zone EV4: attenuation ranges around access points AP17,        AP18 and AP19 with level thresholds chosen to correspond to        corridor C.

It should be noted that, for the implementation of the invention, thevariations of the signal strength boundaries around an access point dueto modifications of the environment (number of stations in the field ofthe access points, number of human bodies, humidity, etc.) can becompensated by providing overlapping attenuation ranges of the accesspoints defining each event zone. Then, even if the attenuation range ofa given access point becomes smaller, a station can still be localizedin the event zone because it is in the field of another attenuationrange.

FIG. 4 is a simplified flowchart of the communication method accordingto the present invention.

The flowchart of FIG. 4 illustrates a loop for the localization and theaccess point selection. Preferably, these two determinations are madewith the same periodicity (in the same loop). According to this example,the first step (block 10, AP MAX SL) consists in establishing acommunication between the station and the central server, in aconventional way, through the access point from which the stationreceives the highest signal level. According to the invention, thesignal level values are stored dynamically only during the loop (block11, SL TABLE) by the station with the identifying addresses of thecorresponding access point it is connected to. Then (block 12, EZCHECK), the received signal level measurement of the access point beingused is compared to the predetermined threshold value(s) stored in adedicated table to define the event zones. These comparisons lead to thelocalization of the station in term of event zone(s) in which thestation is present. If the event zone is the same as in the precedingloop, the processing returns to block 10 for the next periodical check.If the event zone is not the same as in the preceding loop, the stationis affected to a new event zone (block 13, NEW EZ) and the correspondingevent (application) starts. Or if the station is not in a defined eventzone, a pre-determined application or information is affected to thestation.

According to a first embodiment of the present invention, a mobilestation periodically checks the signal levels received from the variousaccess points, and stores in a table the received signal levelmeasurements and the corresponding access point address. Then, theobtained data of access point address and reception levels istransmitted to the central server. The server compares the measuredlevels with the thresholds stored in a table of definition of the eventzones and comprising at least, for each event zone, the access pointaddresses of that zone and the corresponding signal strengththreshold(s). If at least one of the strength measurements is lower thanone configured threshold value, the station is considered located in thecorresponding event zone. The station is identified with its address (IPaddress) which is uniquified on the network and dedicated to thephysical corresponding object (the PC, the mobile phone, the PersonalDigital Assistant, etc.).

Alternatively or in combination, according to the invention:

-   -   an event zone can include one or more access point attenuation        ranges;    -   an access point can be included in more than one event zone;    -   the attenuation thresholds defining the attenuation ranges of        several access points participating to a same event zone can be        different from each other;    -   the attenuation thresholds defining the attenuation ranges of an        access point participating to several event zones can be        different from one event zone to another (for example, event        zone EVB of FIG. 2 can be delimited with a coverage area of        access point AP3 higher or lower (in term of attenuation) than        −48 dB);    -   an event zone can be discontinuous, that is defined by        non-overlapping coverage areas;    -   an event zone can be defined with more than one level threshold        of a same access point (for example, an event zone can be        delimited by two thresholds to define a ring around an access        point);    -   the event zone in which is located a mobile station is        independent to the access point through which this mobile        station communicates with the central server. In some cases, a        mobile station can communicate through an access point which        does not serve to define the event zone in which the station is        located; and    -   only a part of the access points of the environment can be used        to define the event zone(s).

An advantage of the present invention is to localize a mobile stationnot only with respect to the access points with which it communicates.

Another advantage of the invention is that the event zones are notphysically fixed but can be modified as needed or even dynamically byapplications or the station's user. Such a modification of the eventzones only needs modifying the correspondence table stored in thecentral server (or in the associated database) between the access pointaddresses, the event zone identifying data and the signal levelthresholds.

Another advantage of the preferred embodiment of the present inventionis that no structural hardware modification of known access points,mobile stations or central servers is required. Indeed the measuringdevices of the signal strength received from various access points arealready provided in the mobile stations dedicated to communicate withthe systems to which preferably applies the present invention.

The practical implementation of the invention based on the functionaldescription above is in the ability of one with an ordinary skill in theart. In particular, the choice of the number of access points and theirdistribution based on the environment to be covered depends of thenumber and sizes of the event zones.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. Such alteration,modification, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended to be limiting. The invention islimited only as defined in the following claims and the equivalentthereto.

1. A localization method of a mobile station (2) communicating with atleast one central server (1) through a wireless network comprising aplurality of wireless radiofrequency transmitting access points (3),among which a first access point is chosen to perform the communication,comprising the steps of: measuring the signal strengths received by saidstation from the plurality of access points; storing each measuredstrength with an address identifying the corresponding connected accesspoint; comparing said stored strengths to values of a predeterminedtable of signal strength thresholds affected to access points, definingone or more event zones (EZ) each comprising one or more attenuationranges of one or more access points; and considering the station aslocated in a given event zone if the measured strength corresponding toan access point defining that event zone is comprised in the attenuationrange of that access point.
 2. The localization method of claim 1, inwhich said attenuation ranges are a function of the environment and ofthe shape of the event zones (EZ).
 3. A communication method between atleast one mobile station (2) and at least one central server (1) throughradiofrequency transmitting access points (3) to which said station iswireless connectable, comprising the steps of: establishing acommunication between said station and said central server through afirst of said access points from which said station receives the highestsignal strength; comparing the signal strength received by said stationfrom at least one second access point with respect to at least onesignal strength threshold used for defining at least one event zone (EZ)in which at least one specific application of the server is to beavailable if said station is present in said event zone; and makingavailable for said mobile station said specific application if thestation is considered in the event zone.
 4. The method of claim 3, inwhich said station is considered to be in the event zone if the receivedsignal strength is lower than said threshold.
 5. The method of claim 3,in which said station is considered to be in the event zone if thereceived signal strength is higher than said threshold.
 6. The method ofclaim 3, in which the station is considered as being in the event zoneby applying the localization method of claim 1 or
 2. 7. The method ofclaim 3, in which the communication are made according to one of thefollowing unregulated spectrum standard suites: 802.11, 802.11a,802.11b, 802.11e, 802.11f, 802.11g, 802.11h, 802.15.1, 802.15TG2,802.15TG3, 802.15TG4, Bluetooth, Wi-Fi, HiperLAN1, HiperLAN2.
 8. Acommunication system between at least one mobile station (2) and atleast one central server (1) through radiofrequency transmitting accesspoints (3) to which the station is wireless connectable, comprising:means to define, with at least a signal strength threshold of at leastone access point, at least one event zone (EZ) in which at least onespecific application of the server is to be available to the station ifpresent in that zone; means to localize the mobile station with respectto the event zone boundary, based on the signal strength received by thestation from the access points.
 9. The system of claim 8, in which thelocalization of the mobile station is performed by said central server(1) on the basis of a table defining each event zone (EZ) by attenuationrange(s) around one or more access points (3).
 10. The system of claim8, further comprising means to implement the communication method of anyof claim 3 to 7.